I all

I have a simple question. I recorded a session of about 25 minutes and now I am just working on one part of the full session. When I select "Wiew->Fit Project to Windows" Cakewalk show me all the 25 minutes.
 
How can I shorten the Project length?

Thanks for your help

control-arrow

What you want isn't shortening the project at all, you just want to display the section you're working on
 
Like Martin alluded to, ctrl + right arrow will zoom into the project as close as you want and the display will centre on wherever you have the Now Time, unless it's at the beginning of the  project.

How many tracks in the project? If not many, one option could be simply hiding the unwanted parts by slip editing.
Then the track view would only show the relevant parts no matter what your zoom setting is.
That would of course mean that the active part "floats" in the middle of the project at bar 587...

Shorten the project lenght? Ripple Edit.